This invention relates to a method for determining the observation position of a sample when a minute shape of the sample surface is observed, the size is measured or the structure and the organization of the sample are analyzed (all these operations are referred to as the examination of an exterior view), by using a scanning type electron microscope (hereinafter abbreviated to SEM).
However, examination of the exterior view of a sample by using SEM has required a relatively long time, as will be understood below. Namely, the sample is first mounted on a sample stage. Then, a low magnification image of the sample is diplayed on a picture tube through operation of SEM. Next, the sample is moved while observing the low magnification image, to find out a portion to be examined. The position is determined by increasing the magnification factor as well as carrying out other operations as required. Thus, a final magnification with which a minute shape can be observed is reached.
The above-mentioned operations can be classified roughly into two parts: the SEM operation and the sample stage operation. The former consists of adjustment of a SEM image, or adjustment of tint, contrast, brightness and distortion, while the latter consists of finding out an observation position and defining the position at the center of the scan domain of the electron beam. Tint, contrast and brightness, etc. are once adjusted, they need not be adjusted again unless no remarkable variation occurs in the height, physical property and size of the sample. Therefore, the main time-consuming work lies in discovering the observation position and defining the position.
The reason for a large time consumption is as follows. Since it is not possible to discern, with the naked eye, even an approximate position of the sample in a sample chamber, the position is determined usually by observing an SEM image. But, the image is difficult to observe. Whereas SEM can demonstrate the physical relation between the irradiated electron beam and the substance which composes the sample on the screen of the picture tube, the overall contrast is low while the noise arising from statistical fluctuations of the number of trapped electrons is large. The dynamic range of SEM observation is much smaller than that of observation with a conventional optical microscope. Furthermore, the information quantity is small due to the laster scan. No color information is obtainable. It should be noted especially that those parts which could be observed with a high contrast by an optical microscope tend to have only a poor contrast in SEM. These facts are causes for a large time consumption in discovering an observation position.
In addition, these are not only a time loss but also cause a secondary drawback described below. Namely, when an electron beam is irradiated on a sample, contamination (deposition of carbon on the sample surface) and damage (variation in the physical property of the sample) may happen depending on the irradiation energy and the irradiation quantity of the electron beam and the vacuum environment, etc.
There would be no problem, if the sample is used only for observation. But, if the sample is used for further purposes, the influence of the above-mentioned effects can not be neglected. In the prior art method, since the electron beam is irradiated on various parts of the sample surface while the observation position is being sought, the sample is contaminated and damaged.